Show Register values in different encodings

Currently the register values are shown in the variable section as positive integers.
It would be cool to have them show up in HEX or in two complement encoding as well.

Start at the debugger variable section that renders register values and trace how those values are formatted. Define the supported HEX and two's-complement presentations, then verify that registers display the selected encodings alongside or instead of positive integers.

Written by the indexing model from the issue text.